1. Field of the Invention
The present invention relates to an active drive type or passive drive type light emitting display device in which a large number of light emitting elements, for example represented by organic EL (electroluminescent) elements, are arranged, and particularly to a self light emitting type display device in which light emitting elements can be efficiently driven to be lit by controlling a drive voltage supplied from a power supply circuit which is for driving and lighting the light emitting elements based on the forward voltages of the respective light emitting elements.
2. Description of the Related Art
A display employing a display panel constructed by arranging light emitting elements in a matrix pattern has been developed widely. As the light emitting element employed in such a display panel, an organic EL element in which an organic material is employed in a light emitting layer has attracted attention. This is because of backgrounds one of which is that by employing, in the light emitting layer of the EL element, an organic compound which enables an excellent light emission characteristic to be expected, a high efficiency and a long life which make an EL element satisfactorily practicable have been advanced.
The organic EL element can be electrically shown by an equivalent circuit as shown in FIG. 1. That is, the organic EL element can be replaced by a structure composed of a diode element E and a parasitic capacitance element Cp which is connected in parallel to this diode element, and the organic EL element has been considered as a capacitive light emitting element. When a light emission drive voltage is applied to this organic EL element, at first, electrical charges corresponding to the electric capacity of this element flow into the electrode as a displacement current and are accumulated. It can be considered that when the voltage then exceeds a determined voltage (light emission threshold voltage=Vth) peculiar to the element in question, current begins to flow from the electrode (anode side of the diode element E) to an organic layer constituting the light emitting layer so that the element emits light at an intensity proportional to this current.
FIG. 2 shows light emission static characteristics of such an organic EL element. According to these, the organic EL element emits light at an intensity L approximately proportional to drive current I as shown in FIG. 2A and emits light while current I flows drastically when the drive voltage V is the light emission threshold voltage Vth or higher as shown by the solid line in FIG. 2B. In other words, when the drive voltage is the light emission threshold voltage Vth or lower, current rarely flows in the EL element, and the EL element does not emit light. Therefore, the EL element has an intensity characteristic that in a light emission possible region in which the voltage is higher than the threshold voltage Vth, the greater the value of the voltage V applied to the EL element, the higher the light emission intensity L of the EL element as shown by the solid line in FIG. 2C.
Meanwhile, it has been known that physical properties of the organic EL element change due long-term use so that the forward voltage VF becomes higher. Thus, as shown in FIG. 2B, the V-I characteristic of the organic EL element changes in a direction shown by the arrow (characteristic shown by the broken line) due to an actual use time, and therefore the intensity characteristic is also deteriorated. The organic EL element has a problem that variations in initial intensities also occur due to for example variations in deposition at the time of film formation of this element, and thus it becomes difficult to express intensity gradation faithful to an input video signal.
Further, it has been known that the intensity property of the organic EL element changes due to changes in environmental temperature roughly as shown by broken lines in FIG. 2C. That is, while the EL element has the characteristic that the greater the value of the voltage V applied thereto, the higher the light emission intensity L thereof in the light emission possible region in which the voltage is higher than the light emission threshold voltage as described above, the EL element also has a characteristic that the higher the temperature becomes, the lower the light emission threshold voltage becomes. Accordingly, the intensity of the EL element has a temperature dependency that the higher the temperature becomes, the lower the applied voltage by which light emission becomes possible and that the EL element is brighter at a high temperature time and is darker at a lower temperature time though the same light emission possible voltage is applied.
In general, a constant current drive is performed for the organic EL element due to the reason that the voltage vs. intensity characteristic is unstable with respect to temperature changes while the current vs. intensity characteristic is stable with respect to temperature changes, the reason that it is necessary to prevent the EL element from being deteriorated by an excess current, and the like. In this case, an operational voltage VH, for example produced from a DC/DC converter or the like, which is supplied to a constant current circuit, has to be set, considering the following respective factors.
That is, as the factors, it is possible to enumerate the forward voltage VF of an EL element, a variation part VB of the VF of an EL element, a change-with-time part VL of the VF, a temperature change part VT of the VF, a drop voltage VD necessary for allowing a constant current circuit to perform a constant current operation, and the like. Even when these factors interact synergistically, in order to fully ensure the constant current characteristic of a constant current circuit, the operational voltage VH has to be set at a value obtained by adding maximum values of respective voltages shown as the respective factors.
However, a case where a voltage value obtained by adding maximum values of respective voltages as described above is needed as the operational voltage VH supplied to the constant current circuit hardly occurs, and in a usual state, a large power loss as a voltage drop part in the constant current circuit is brought about. Therefore, this becomes a primary factor of generation of heat, thereby putting stress on organic EL elements, peripheral circuit parts, and the like.
Japanese Patent Application Laid-Open No. H7-36409 (paragraphs 0007 to 0009 and FIG. 1) discloses a countermeasure for dissolving the above-described problems by measuring the forward voltage VF of an EL element and by appropriately controlling the value of the operational voltage VH given to the constant current circuit based on this VF.
In the structure disclosed in Japanese Patent Application Laid-Open No. H7-36409 (paragraphs 0007 to 0009 and FIG. 1), the forward voltage VF of one light emitting element (EL element) arranged in a display panel is detected so that an operational voltage given to a constant current circuit which drives respective light emitting elements is controlled based on the forward voltage of this light emitting element. FIG. 3 shows such a structure in a simple way, wherein reference numeral 1 designates a constant current circuit, and reference numeral 2 indicates a light emitting element represented by an organic EL element whose light emission is controlled by the constant current circuit 1. This structure is constructed in such a manner that the forward voltage VF of the light emitting element 1 generated by supplying constant current from the constant current circuit 1 to a light emitting element 2 is detected by a forward voltage detection circuit 3 so that a detection output by this voltage detection circuit 3 is sent to a comparison/calculation circuit 4.
A voltage setting circuit 5 generating a predetermined voltage (reference voltage) that is a comparison object is connected to the comparison/calculation circuit 4. In the comparison/calculation circuit 4, the reference voltage supplied from the voltage setting circuit 5 and a voltage corresponding to the forward voltage VF supplied from the voltage detection circuit 3 are compared to generate a control voltage corresponding to the difference part of these voltages. The control voltage corresponding to the difference part is supplied to a voltage boost circuit 6 for example made of a switching regulator as a power supply circuit to control the value of an operational voltage (power supply voltage) VH outputted from the voltage boost circuit 6.
In the structure shown in FIG. 3, where the reference voltage supplied from the voltage setting circuit 5 is “Vconstant,” the value of the operational voltage VH is controlled so as to follow the relationship of “VH=VF+Vconstant.” The operational voltage VH controlled in such a way operates to so as to constant current control the constant current circuit 1, whereby the light emitting element 2 is constant current driven. Therefore, the operational voltage VH which constant current controls the constant current circuit 1 is controlled so as to be changed, taking a voltage margin of the “Vconstant” accompanied by changes of the forward voltage VF of a light emitting element. Accordingly, a voltage drop part generated in the constant current circuit 1 can be restricted within a certain range, and a power loss generated in the constant current circuit 1 can be reduced.
In the structure shown in FIG. 3, as already described, the forward voltage VF of one light emitting element (EL element) arranged in a display panel is detected, and based on this forward voltage, the value of the operational voltage VH given to the constant current circuit which drives respective light emitting elements is controlled. Accordingly, for example as shown in FIG. 4, in a case where a wiring line of the anode side or the cathode side of the light emitting element 2 which becomes a detection object of the forward voltage VF is cut, or in a case where the light emitting element 2 is destroyed or the like, the forward voltage VF is deemed to be raised to an extremely high level of voltage. As a result, the operational voltage VH outputted from the voltage boost circuit 6 provided as a power supply circuit is extremely raised, and a problem that the circuit is damaged or is broken in an extreme condition by the boosted operational voltage VH may develop.